A defender or midfielder, most comfortable at left-back but able to play in a range of positions, he began his career with his hometown club, Carlisle United, signing professional forms on 31 March 1989.

At this time Liverpool were still the dominant side in England as holders of the FA Cup and having not finished lower than champions or runners-up in the First Division since 1981.

[1] Harkness suffered a broken leg in Liverpool's 1–0 league defeat at Coventry City[2] on 6 April 1996 and was out of action for nearly a year afterwards.

He missed the 1996 FA Cup Final, which Liverpool lost 1–0 to Manchester United, and also missed the bulk of the 1996–97 season, in which Liverpool frequently led the Premier League until being leapfrogged by Manchester United in late January and were ultimately unable to stop their north-west rivals from retaining the league title.

Stan Collymore alleged that during a 1998 match for Aston Villa against Liverpool, Harkness subjected him to a 10-minute barrage of insults pertaining to his mixed-race heritage.

On 1 November that year, aged 31, having played just 10 times due to ankle injury, he ended his contract with the Conference Premier club by mutual consent.

He was sentenced to a three-year driving ban, a nine-week jail term suspended for two years, and 100 hours of community service.